Do I need a hard drive to play dawngaurd off line and how much space does it take?
The Dawnguard DLC is around 500-600 MB, you have more than enough space for the download. The guy that said it was 3.8 gb is mistaken, i believe he's thinking of the Skyrim install size which is about right. Good luck

you won't be disappointed, it's a great add-on